The fact is that this person did not use the word alien in their original "joke" - they wrote "That's assuming Einstein was even human." They then proceeded to act like a bully or abuser would act - "Oh, it was a joke! I meant it as a joke and I forgot to drop the punchline until almost an hour later! You are so dumb for thinking otherwise!" As an older sibling, this is something straight out of older sibling 101 and I picked up on it immediately when I saw the interaction between sillysaurusx and oldstrangers.

There is no weird conspiracy here, there was just someone acting in an uncivil manner who was called out on it and who then kept things going.

I was actually expecting the joke to fall back on the fact that he was indeed a completely legal alien. Because he was, and that is humor.